Graft-versus-host (GVH) alloresponses mediated by donor lymphocyte infusions (DLI) administered to[unreadable] established murine mixed hematopoietic chimeras eliminate normal and malignant host hematopoietic cells[unreadable] without causing graft-versus-host disease (GVHD). Administration of similar DLI immediately following[unreadable] conditioning leads to severe GVHD. More potent graft-versus-tumor (GVT) effects are achieved from[unreadable] delayed DLI given to mixed hematopoietic chimeras than are achieved in fully allogeneic chimeras. We have[unreadable] observed that GVH-reactive T cells are activated, expand, produce cytokines and adopt the "memory"[unreadable] phenotype when administered as DLI to established mixed allogeneic chimeras. Therefore, the lack of[unreadable] GVHD cannot be attributed to global suppression of the alloresponse by regulatory cells. This potent GVH[unreadable] reaction is largely confined to the lymphohematopoietic system, as T cells do not accumulate in GVHD target[unreadable] tissues. We refer to this as a lymphohematopoietic GVH reaction (LGVHR). We have obtained evidence[unreadable] that a similar phenomenon can occur clinically when DLI are given to patients in whom mixed chimerism is[unreadable] established with non-myeloablative conditioning. The ability to achieve LGVHR without GVHD across MHC[unreadable] barriers provides an approach to achieving powerful GVT effects against lymphohematopoietic malignancies[unreadable] without GVHD. In order to identify the mechanisms whereby GVHR are confined to the[unreadable] lymphohematopoietic system following DLI, we will: 1) Compare the kinetics of GVH-reactive CD4 and CDS[unreadable] T cell activation, proliferation, differentiation, tissue accumulation and death in mice receiving DLI[unreadable] immediately following irradiation or with a delay following establishment of mixed allogeneic chimerism; we[unreadable] will address the significance and mechanisms of increased apoptosis and other differences in mixed[unreadable] chimeras vs freshly irradiated mice. 2) Compare the survival, proliferation, migratory properties and GVHD[unreadable] effector function of effector/memory T cells generated in a non-inflammatory environment (i.e. following DLI[unreadable] administration to established mixed chimeras) versus a pro-inflammatory environment (i.e. following DLI[unreadable] administration to freshly irradiated mice) when the cells are adoptively transferred to either type of[unreadable] environment. These studies will determine the extent to which T cell-intrinsic versus extrinsic environmental[unreadable] factors determine the capacity of a given effector/memory cell population to induce GVHD; 3) Examine the[unreadable] role of regulatory cell populations and of T cell homeostatic proliferation in modulating susceptibility to[unreadable] GVHD. The studies will synergize with those in Projects 2 and 3, and will make extensive use of Cores A, B,[unreadable] and C. An improved understanding of the mechanisms of LGVHR in delayed DLI recipients will advance the[unreadable] studies in Project 3 and ultimately the clinical use of this approach to separating GVHD and GVT effects in[unreadable] HLA-mismatched hematopoietic cell transplantation.